Usually a torque converter or other fluid transmission mechanism is arranged between an automatic transmission and an engine to boost the torque. Because the torque converter performs power transmission via a fluid, however, the transmission efficiency is poor, and this is undesirable.
In Japanese Kokai Patent Application No. 2003-035353, which is assigned to the assignee of the present invention, a type of input rotation transmission mechanism with a torque boosting function and having a high transmission efficiency is proposed. That input rotation transmission mechanism is composed of an epicyclic gear mechanism instead of a fluid transmission mechanism. It has a low-speed stage that reduces the input engine speed and a high-speed stage that transmits the input engine rotation as is. Because up-shifting and down-shifting are performed in the low-speed stage and high-speed stage, respectively, this input rotation transmission mechanism is also called a secondary transmission. As a substitute for the torque converter, it is inserted in the drive train between the engine and the automatic transmission, also known as the principal transmission. When the vehicle starts to move, the input rotation transmission mechanism is in the low-speed stage selection state, and from the neutral state in which the driving frictional elements of the back/forth switching mechanism of the automatic transmission are released, driving is effected by control to engage the driving frictional element.
In Japanese Kokai Patent Application No. 2003-269600, which is also assigned to the assignee of the present invention, there is taught a method of control for preventing gear-shifting shock by means of gear-shifting (shifting operation) with synchronization between the input rotation transmission mechanism and the automatic transmission.